1. Field
The invention relates to secondary batteries, and in particular, to flexible secondary batteries.
2. Description of the Related Art
A recent development of electronic technology has led to rapid expansion of markets for various electronic devices, such as smartphones, smart pads, electronic books, watch-shaped telephones and mobile medical devices, which may be bonded to the human body, in addition to mobile phones, game players, portable multimedia players (“PMP”s) and MPEG audio layer-3 (“MP3”) players. Due to the growth of the markets for such mobile electronic devices, a demand for batteries that are suitable for driving the mobile electronic devices is increasing.
In particular, for convenience during use of mobile electronic devices, a demand for flexibility of a battery is gradually increasing. Also, for ease of installation of a battery in a limited space of the mobile electronic devices, a required flexibility level of a battery is further increasing.
If a battery that is not sufficiently flexible is bent, a stress may be focused on, for example, an interface of a separator and electrode active material layers of two electrodes, an inner interface of an electrode active material layer, or an interface of an electrode active material layer and a current collector, thereby causing peeling. This may negatively affect performance and lifespan of the battery.
A typical and commercially available lithium ion secondary battery may have a structure of current collector-positive active material layer-separator-negative active material layer-current collector. If a battery having such a structure is repeatedly deformed, relative positions of reacting electrodes or an interval between electrodes may be changed, thereby leading to an unstable electrochemical reaction that may cause a decrease in performance of a battery, and more seriously, a short.